This invention relates to a textile sheetlike material for covering a pasty active mass on a planar plate-shaped battery electrode. This invention further relates to a planar plate-shaped battery electrode comprising such a material, a battery and also a process for producing such battery electrodes.
It is customary to use lead batteries in vehicles as rechargeable electric energy stores or specifically as starter batteries. A lead battery typically includes positive and negative electrodes and also a dilute solution of sulfuric acid as electrolyte. The electrodes are each coated with an active mass. Lead dioxide can be used as active mass for a positive electrode for example, while spongy lead can be used as active mass for the negative electrode for example. The active mass is applied to the plate-shaped electrodes in pasty form.
Stackwise storage of the plate-shaped electrodes coated with the pasty active mass requires that sticking together of the electrodes must be avoided. Moreover, the relatively sensitive active mass also has to be mechanically stabilized. For this purpose, it was hitherto customary to apply to the active mass a covering layer of paper. The use of paper as covering layer, however, has various disadvantages, including the cellulose fibers of the paper dissolving in the sulfuric acid in operation of the battery and hence no longer being able to contribute to stabilizing the active mass.
To improve this situation, WO 03/007404 A1 proposed forming the covering layer of a fibrous nonwoven web, preferably a polyester nonwoven. However, it has transpired in the meantime that such a polyester nonwoven likewise does not have long durability in operation of the battery. For instance, the battery may occasionally be overcharged in practice. Relatively high temperatures can arise, which as a result of the oxygen forming at the positive electrode leads to destruction of the polyester nonwoven. The polyester nonwoven disintegrates, which has the consequence that the polyester fibers rise to the surface of the liquid electrolyte. Electrically conducting bridges can form together with impurities adhering to the fibers, and lead to shortcircuiting between the positive and negative electrodes and hence to outage of the battery.
It has also already been proposed, by WO 98/01914 A1 for example, to coat the active mass of the electrodes with glass microfiber. However, it has transpired that the veils of glass fibers have an excessively open structure wherethrough the pasty active mass was able to squeeze, which leads to contamination of the electrolyte and hence to outage of the battery. In addition, the glass fiber veil proved to be problematical because of its waviness and deficient breaking strength in the wet state. Therefore, the use of glass fiber veil has also not been successful in practice.
It is an object of the present invention to provide ways to improve lead batteries that are fitter for purpose in practice and stabilize the pasty active mass more securely on the battery electrodes.
This object is achieved by the material recited in claim 1. The object is further achieved by a battery electrode according to claim 8, a battery according to claim 10 and a process according to claim 14. The dependent claims recite advantageous embodiments of the invention.
The present invention relates to the field of batteries comprising planar, plate-shaped battery electrodes. Such battery electrodes include a frame composed of the electrode material and also a grid produced therein from the electrode material. Such a battery electrode has a pasty active mass applied to it. Batteries comprising electrodes thus constructed are used in particular as starter batteries for vehicles. In contrast for example to batteries that include so-called tubular electrodes, the construction of which is described in DE 36 436 43 C2 for example, and which can be used in particular as traction batteries for electrovehicles, plate-shaped battery electrodes need to be stackable on top of each other for storage in the course of the manufacture of batteries. The tubular electrodes mentioned do not have such a need for example, since there is no stackable storage involved with them and they generally have a different construction in that they do not contain a grid for example.
The invention has the advantage of mechanically stabilizing the pasty active mass in the battery and of preventing the mass being washed away into the electrolyte. This makes it possible to produce cycle-proof batteries having a long durability. Using the invention material with a mixture of glass fibers and fibers composed of a thermoplastic polymer ensures that a covering layer formed therefrom for the pasty active mass will have durability throughout the entire battery life. As a result, there is in particular no need to apply an additional layer of paper. The invention material for covering the pasty active mass is also mechanically appreciably more stable than the prior art materials during the production process of the electrodes, i.e., in the course of application to the battery electrode. The material of the invention is readily fixed to the battery electrode and cut. Stacking the pre-fabricated battery electrodes is now also possible without damaging the material of the invention, more particularly there is no squeezing of active mass through the covering layer. Even after a storage operation involving the pre-fabricated battery electrodes, the material of the invention exhibits improved adhesion to the battery electrode compared with the prior art materials.
Nor does the textile sheetlike material of the invention disintegrate in operation of the battery equipped with the battery electrode of the invention, as is the case with the prior art materials for example. Therefore, stability of the covering layer is ensured throughout the battery life. The textile sheetlike material of the invention also does not disintegrate during the process of formation.
In an advantageous development of the invention, polyester fibers are used as fibers composed of a thermoplastic polymer. This allows inexpensive production of the textile material. In addition, polyester fibers are readily available.
In an advantageous development of the invention, the glass fibers and/or the fibers composed of a thermoplastic polymer are finely fibrous. It is more particularly advantageous for the fibers to have a diameter of less than 6 μm. This makes it possible to realize a relatively finely fibrous textile structure and thus to avoid the pasty active mass passing (squeezing) through the textile material.
In an advantageous embodiment of the invention, the fibers composed of a thermoplastic polymer have a weight fraction of 5 to 25 percent, based on the material. It has been determined that this makes it possible to achieve particular robustness on the part of the material according to the present invention.
In an advantageous development of the invention, the textile material, in addition to the glass fibers and the fibers composed of a thermoplastic polymer, includes a binder to consolidate the fibers. This makes it possible to further enhance the mechanical stability and sustained use strength of the material.
In an advantageous development of the invention, an acid-resistant binder, more particularly an acrylate binder, is provided. This further enhances the sustained use strength of the material.
In an advantageous development of the invention, the binder has a weight fraction of 5 to 25 percent, based on the material. The further fractions of the textile material of the present invention consist of the glass fibers and optionally further additives. Advantageously, the predominant weight fraction is formed by the glass fibers. In an advantageous development of the invention, the textile material of the present invention can consist of the mixture of glass fibers and fibers composed of a thermoplastic polymer with or without the binder.
The textile sheetlike material of the present invention is suitable for the planned purpose of coating battery electrodes, more particularly in material thicknesses of 0.2 to 0.3 mm, for example 0.25 mm.
In an advantageous development, a planar, plate-shaped battery electrode includes a planar electrode grid arranged in a frame, wherein the electrode grid is coated with a pasty active mass and the active mass is covered with a covering layer of a textile sheetlike material according to any preceding claim.
In an advantageous development of the invention, a battery includes at least two electrodes, the pasty active mass of which has a covering layer of the material of the present invention applied to it. Furthermore, the electrodes are insulated from each other by interposition between the electrodes of a separator which, in an advantageous embodiment, includes glass fibers.
The separator can also consist wholly of glass fibers. The glass fiber fraction in the invention textile sheetlike material of the covering layer provides a synergistic effect by improving the contact between the covering layer and the separator compared with prior art materials for the covering layer, thereby improving the contact of the electrodes with the electrolyte. This increases the efficiency of the battery.